Photonic sensing apparatus can derive significant benefits from the ability to provide a response over more than a single range of wavelengths. In recognition of this, several constructions have been proposed to provide photonic sensing apparatus multi-spectral functionality.
In one construction, photosensitive layers having divergent radiation absorption profiles are presented in a stacked configuration to provide multi-spectral functionality, wherein photosensitive layers absorbing shorter wavelengths of radiation are positioned over photosensitive layers absorbing longer wavelengths of radiation to avoid potential attenuation and interference of short wavelength radiation by long wavelength photosensitive materials. Long wavelength photosensitive materials, for example, can absorb and/or scatter radiation of shorter wavelengths, thereby providing significant attenuation or even precluding such radiation from reaching a short wavelength photosensitive layer.
Restriction to short wavelength photosensitive layers disposed over long wavelength photosensitive layers in a stacked photodetector configuration presents several disadvantages. One disadvantage is degradation of long wavelength photosensitive materials and structures resulting from the processing required for the subsequent deposition of one or more short wavelength photosensitive materials.
Another disadvantage is the inability to thermally isolate long wavelength photosensitive layers in prior stacked configurations. Materials absorbing long wavelength electromagnetic radiation are often sensitive to thermal fluctuations and can suffer significant signal to noise (S/N) degradation resulting from dark currents in response to such thermal fluctuations. As a result, long wavelength photosensitive layers often require thermal isolation in order to provide the desired response.
As an alternative to photosensitive layers presented in a stacked configuration, photodetector constructions based on a plurality focal plane arrays have been proposed. In such constructions, each focal plane array is sensitive to radiation of a different spectral region. Optical assemblies are used to direct and distribute light received by the detector to the plurality of focal plane arrays. The various complexities of the optical assemblies used in directing the received light, nevertheless, are a significant disadvantage of multiple focal plane constructions.
A similar disadvantage is encountered in the use of segmented focal plane arrays wherein each segment of the array is sensitive to electromagnetic radiation of a different spectral region. As with multiple focal plane array constructions, use of segmented focal plane arrays usually requires complex optical assemblies to ensure the correct distribution of received light over the segmented array.